In a gas turbine engine, hot combustion gases generally may flow from one or more combustors through a transition piece and along a hot gas path of a turbine. A number of turbine stages typically may be disposed in series along the hot gas path so that the combustion gases flow through first-stage nozzles and buckets and subsequently through nozzles and buckets of later stages of the turbine. In this manner, the nozzles may direct the combustion gases toward the respective buckets, causing the buckets to rotate and drive a load, such as an electrical generator and the like. The combustion gases may be contained by stationary circumferential shrouds surrounding the buckets, which also may aid in directing the combustion gases along the hot gas path.
Certain turbine buckets may include a tip shroud positioned radially outward from an airfoil thereof. During operation of the turbine, the tip shroud may prevent failure of the airfoil in high cycle fatigue due to vibratory stresses. However, stresses may be induced at a fillet region between the airfoil and the tip shroud due to centrifugal forces acting on the tip shroud. According to certain configurations, the turbine bucket also may include a seal rail positioned radially outward from the tip shroud and extending in a tangential direction with respect to a central axis of rotation of the turbine. The seal rail generally may extend radially into a groove formed in the corresponding stationary circumferential shroud. In this manner, the seal rail may control or prevent leakage of the combustion gases between the tip shroud and the stationary circumferential shroud. Further, the seal rail may reduce bending of the tip shroud, although the added mass of the seal rail may increase stresses at the fillet region.
According to one known configuration, the seal rail may extend in the tangential direction from a first end to a second end of the tip shroud, and the seal rail may have an axial thickness that is constant along the tangential direction. Although such configuration may control leakage over the tip shroud and may reduce bending of the tip shroud, the added mass of the seal rail, particularly at the ends of the tip shroud, may significantly increase stresses at the fillet region. Increased stresses at high operating temperatures may lead to a high creep rate on the tip shroud, which may reduce part life of the turbine bucket. Additionally, the increased stresses at elevated temperatures may reduce fatigue life of the turbine bucket. Further, such a seal rail configuration may present challenges in achieving tip shroud balance and frequency tuning of the turbine bucket, which also may reduce part life of the turbine bucket.
There is thus a desire for a turbine bucket having an improved seal rail configuration for achieving tip shroud balance and frequency tuning of the turbine bucket. Specifically, such a seal rail configuration may be optimized to achieve proper tip shroud balance while also providing the necessary seal rail mass for supporting the tip shroud and maintaining desired frequency margins. In this manner, such a seal rail configuration may increase part life of the turbine bucket and thus may reduce the incidence of costly repairs and shutdown of the turbine.